One- and two-year droughts recur in January 2019

Drought contour plot to Jan 2019

This contour plot shows the progress of the extreme drought at Manilla up to January 2019. Colours show rainfall shortages as percentiles. Dates plot along the top, and durations down the side.

One month rainfall totals (on the top row)

By January 2019, there had been six months without serious monthly rainfall shortages. The months of serious rainfall shortage (light brown) were earlier, in May, June and July 2018. The only other month with such low rainfall was September 2017.

Droughts lasting less than one year (rows 2, 3, 4, 5, 6, 9)

As the effects of low monthly rainfall added up, extreme droughts appeared (dark brown). That is, rainfall totals below the 1st percentile. (They are the lowest rainfall totals, that have occurred in less than 1% of the historical record.)
By June 2018, the 2-month and 3-month totals were already extreme shortages. Similarly, by July, the 3-month, 4-month, and 5-month totals were extreme shortages. By September 2018, extreme shortages extended as far as the 9-month total. That total, adding up the nine months from January to September 2018, included only one month (February) that had rainfall above normal.
In these durations of less than one year, extreme droughts were rare after September 2018. Because the last six months had no serious monthly shortages, the final month (January) includes no serious shortages for durations from 2 months up to 6 months.

Droughts of 1 year to 2 year duration (rows 12, 15, 18, 24)

By August 2018, an extreme 15-month drought appeared. That 15-month total then included not only the dry months of winter 2018, but also the dry month of September 2017. By September 2018, the 15-month total became the driest on record (400 mm). By October all four droughts in this group (12-, 15-, 18-, and 24-month droughts) were extreme. This became true again in January 2019. By that date, some of the dry months of 2017 were no longer included, but dry months in the current summer replaced them in the total.


Related graphs

A similar contour plot with data to October 2018 is here.

A line graph of the rainfall status for January 2019 reveals that extreme shortages also exist now at the much longer durations of six years and seven years that are not included in this contour plot.


Notes

The following notes include:

Classes of rainfall shortage

Rainfall rate versus percentile rank

Limitations of this analysis

Note: Classes of rainfall shortage

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The 2002 drought contour chart

Contour chart 2002 drought at Manilla NSW

The 2002 drought at Manilla was a failure of winter rainfall. [See Note below: “Manilla’s rainfall seasonality”.]

The top line of this contour chart shows that monthly rainfall shortages occurred in all the six months of winter rainfall dominance (April to September) of 2002. Shortages in May and July were severe, below the 5th percentile. In the summer rainfall months (October to March) that preceded and followed, rainfall was near or above normal. [See Note below: “Classes of rainfall shortage”.]

Lower down the contour chart, rainfall shortages of longer duration are shown. For droughts of 3 months duration, the rainfall shortage was extreme (1st percentile) by July 2002, as it included the serious shortage of May as well as that of July. In the same way, one sees extreme 6-month shortages in September and October, as all the monthly rainfall shortages since March added up.

By November 2002, one sees extreme droughts of 9 months and 12 months duration. The 9-month drought incorporated the consecutive months of below-normal rainfall from March to November. The extreme 12-month drought (307 mm) that was evident in November 2002 began earlier, with below-normal rainfalls in December 2001 and January 2002. That was the fourth driest 12-month period on record, after October 1965 (288 mm), August 1946 (302 mm) and November 1965 (304 mm).

The 2002 drought had no extreme rainfall shortages longer than 12 months. There were, however, some severe shortages of 18 months duration and some serious shortages of 24 months duration, due to some low rainfalls in the previous winter (2001).

By April 2003, hardly any serious rainfall shortages due to the 2002 drought remained. [See Note below: “Limitations of this analysis”]

More about the 2002 drought

Graph of monthly percentile rainfall in a droughtAnother approach to describing this 2002 drought is in the post “The 2002 rainfall shortages at Manilla”. That post has a graph showing selected monthly profiles of percentile values. It also links to two earlier posts with graphs of smoothed values of climatic anomalies.

The 2018 Drought

Drought 2018 contour chartA similar contour plot for the drought of 2018 reveals similarities and differences.


Note: Classes of rainfall shortage

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Contours of Manilla’s 2018 Drought

Drought 2018 contour chart

Displaying drought severity, date, and duration.

This image shows how an extreme drought developed at Manilla, NSW in 2018. The contours and colours indicate drought severity. They show whether the rainfall shortage was extreme, severe, or merely serious. At times without drought, they show when the rainfall was above normal.
[See Note below: “Classes of rainfall shortage”.]
Calendar months appear in order along the top, showing how the severity of drought has changed as time has passed.
Down the side, the duration of drought is shown, from one month duration at the top to thirty-six months duration at the bottom. Each month is shown on one vertical line, with a severity value plotted against each duration value.
[In other posts (like “Record 15-month Drought in 2018”) each individual month’s drought data is plotted as severity versus duration.]

The pattern of the 2018 drought

Development

In each of the months May, June, and July 2018, the monthly rainfall was a serious or severe shortage, below the 10th percentile. As rainfall had been below normal also in March and April, each month after April saw severe and even extreme shortages that extended to durations of 3 months and longer. By August, there were extreme droughts of five-month and six-month duration, despite reasonable rain (28 mm) having fallen in August.
By September, the nine months to date were in extreme drought. That is to say, the first nine months of 2018 had a total rainfall lower than any but 1% of all nine-month periods in history.
However, in that month (September 2018), the rainfall shortage for a 12-month duration was not extreme, but merely severe. Yet the 15-month shortage for that month was also extreme. In fact, it was the lowest 15-month rainfall total in history (400 mm).
The source of the extreme 15-month shortage at this date is obvious. One year earlier, the month of September 2017 had a serious rainfall shortage, and the two previous months had little rain. From these short-term shortages in spring 2017, shortages of longer duration descend as an arc across the graph. They become mild nine-month shortages during the summer, then worsen to extreme by merging with 2018 shortages.

Seasonality

Manilla’s rainfall is seasonal, with two distinct modes, each dominating half the year. There is a major summer (monsoonal) mode and a minor winter (westerly) mode. The summer rainfall mode dominates from October to March and the winter rainfall mode dominates from April to September. [See Note below: “Manilla’s rainfall seasonality”.]
This graph documents a failure of winter rainfall. The 2018 months that had rainfall shortages were the months of dominance of the winter rainfall mode (April to September). Rainfall had been near or above normal in the preceding summer rainfall mode (October 2017 to March 2018), and was so again in October 2018. [See Note below: “Limitations of this analysis”]
The rainfall shortages in the previous year (2017) were also restricted to the months of the winter rainfall mode, but began very late. Shortages did not develop until the final three months (July, August, September). Only when the extreme drought of 2018 developed did those 2017 shortages have a big effect.


The 2002 drought contour plot

Contour chart 2002 drought at Manilla NSWThe most recent extreme drought at Manilla before 2018 was the drought of 2002.
A contour plot in the same format is in this post.


Note: Classes of rainfall shortage

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